1. Field of the Invention
The present invention relates to a system for controlling a driving force in a hybrid vehicle which is provided as its prime mover with an internal combustion engine such as a gasoline engine or a Diesel engine and an electric motor such as a motor/generator for outputting a torque when energized by an electric power. More particularly, the invention relates to a system for controlling the driving force of the hybrid vehicle in which a transmission having a gear stage to be set by applying a one-way clutch is arranged a power transmission line leading from the prime mover to drive wheels.
2. Related Art
As the type of the aforementioned hybrid vehicle employing the internal combustion engine and the electric motor as its prime mover, there is known a series hybrid type using the internal combustion engine only for the electric power generation or a parallel hybrid type using the internal combustion engine for both the power generation and the running power. In the latter parallel hybrid type, the vehicle is run by the output of the internal combustion engine transmitted to the drive wheels. As a result, the driving power requested for the run and the output characteristics of the internal combustion engine are not necessarily coincide, and the internal combustion engine cannot be rotated backward. This makes it preferable to provide a transmission as in the general automobile of the prior art.
One example of the hybrid vehicle, which is equipped with an automatic transmission capable of setting a plurality of gear stages, is disclosed in Japanese Patent Laid-Open No. 9-193676 (JPA9-193676). The hybrid vehicle, as disclosed, is constructed such that a motor/generator is coupled to a torque synthesizing/distributing mechanism including a planetary gear mechanism and such that the engine is selectively coupled to the torque synthesizing/distributing mechanism through an input clutch. The hybrid vehicle is equipped with a power transmission line which has a plurality of gear stages coupled to the output side of the torque synthesizing/distributing mechanism. This automatic transmission is constructed like the known automatic transmission such that a low or medium gear stage is set by applying a one-way clutch so as to facilitate the change of the speed at the low or medium gear stage.
As well known in the art, the one-way clutch is application means which is applied/released according to the acting direction of a torque. When the speed is changed from a gear stage, which is set by applying the one-way clutch, to another, the direction of the torque to act on the one-way clutch is inverted according to the change in the applied/released state of a frictional engagement device such as a multi-disc clutch or a multi-disc brake. As a result, the desired speed change is achieved as the one-way clutch is automatically released. Since the release of the one-way clutch is effected by the inversion of the direction of the torque acting thereon, the direction of the torque to act on the one-way clutch is inverted in the deceleration of the vehicle from that of the driving state so that the one-way clutch comes into an overrun state or a released state.
When a down shift to a gear stage to be set by applying the one-way clutch is caused by depressing an accelerator pedal again from a deceleration state (or a coasting state) with the accelerator pedal being released, for example, the one-way clutch, as released in the coasting state, is applied by the so-called "power ON downshift". In this case, the speed of a rotary member on an input side with respect to the one-way clutch rises from an engine speed (or an input speed) to a synchronous speed at the gear stage after the speed change.
As a result, the difference in the speed before and after the speed change increases to raise the accompanying inertial force, so that the shock at the time of applying the one-way clutch may deteriorate. Since the speed of the rotary member on the input side changes highly, the time period for the gear stage after the speed change to reach the synchronous speed is elongated to cause an increase in the driving force or the so-called "sluggishness", as exemplified by a delay in the acceleration.